Quoted from vid1900:Red, Orange and Green LEDs are usually 2v or so, but Blue might be 3 or 4v or higher.
There are many online LED resistor calculators to find the resistor value for the oddball LEDs.
http://led.linear1.org/led.wiz
Looks like you are you basically running two sets LEDs, each in a 2 LED series, no? If so, with a 6.3V input, if you had LEDs with a 3-3.2V forward voltage (blue for example), you wouldn't need any resistor in this setup. If you dropped to 5V DC, depending on the color used, you may have to do all 4 in parallel as you wouldn't have enough supply to feed 2 of them in series.
Definitely use the calculator link. Placing the resistor before or after the last LED really doesn't matter, but I like to put them before because in my brain the power gets dissipated before reaching the LED (not really true).